1. Field of the Invention
The present invention relates to a ribbon cassette for a thermal transfer printer and, more particularly, a ribbon cassette capable of stabilizing the movement of an ink ribbon.
2. Description of the Related Art
Generally, a thermal transfer printer presses a thermal print head mounted on a carriage through an ink ribbon and a recording medium, such as a paper sheet, against a platen, moves the carriage along the platen, drives the heating elements of the thermal print head selectively according to print data to transfer the ink of the ink ribbon to the recording medium in order that a desired image is printed on the recording medium. Since thermal transfer printers are capable of silently printing images in a high print quality, can be manufactured at low costs and are easy to maintain, thermal transfer printers are used prevalently as output devices for computers and word processors.
FIG. 4 is a plan view showing a main part of a conventional ribbon cassette used in such thermal printers.
Referring to FIG. 4, a take-up reel 4 for taking up an ink ribbon 3 and a feed reel 5 holding the coiled ink ribbon 3 are supported for rotation in a housing 1 having an upper case, not shown, and a lower case 2. A print head receiving part 6 is formed in a front portion of the housing 1 to be disposed opposite to the platen of the thermal transfer printer when the ribbon cassette is put in place on the carriage of the thermal transfer printer. When the ribbon cassette is put in place on the cartridge, a thermal head included in the thermal transfer printer is received in the print head receiving part 6. A take-up pinch roller 7 and a feed pinch roller 8 are supported for rotation at a position between the print head receiving part 6 and the take-up reel 4 and at a position between the print head receiving part 6 and the feed reel 5, respectively, on the housing 1. The take-up pinch roller 7 and the feed pinch roller 8 help the ink ribbon 3 move. Six guide rollers 9, 10, 11, 12, 13 and 14 are supported for rotation on pins projecting from the upper case and the lower case 2 to form a passage for the ink ribbon 3.
Each of the take-up reel 4 and the feed reel 5 is formed in a substantially cylindrical shape and has a bore provided with a plurality of splines. When the ribbon cassette is put in place on the carriage of the thermal transfer printer, a take-up shaft and a feed shaft are engaged with the take-up reel 4 and the feed reel 5, respectively. Each of the take-up pinch roller 7 and the feed pinch roller 8 is formed in a substantially cylindrical shape and has a bore provided with a plurality of splines, and when put in place on the carriage, they are engaged with a drive shaft and a tension shaft projecting from the carriage. The take-up pinch roller 7 and the feed pinch roller 8 have sleeves formed of an elastic, relatively highly frictional material. The guide rollers 13 and 14, and the guide rollers 9 and 10 are disposed so that the ink ribbon 3 wraps around the take-up pinch roller 7 and the feed pinch roller 8 at a contact angle in the range of 110.degree. to 180.degree.. The take-up pinch roller 7 is driven for rotation by the drive shaft of the thermal transfer printer to separate the used part of the ink ribbon 3 from the recording sheet. The feed pinch roller 8 is controlled by the tension shaft of the thermal transfer printer so as to apply a tensile load to the ink ribbon 3 to stabilize the movement of the ink ribbon 3.
The passage of the ink ribbon 3 extends from the feed reel 5 via the guide roller 9, the circumference of the feed pinch roller 8, the guide rollers 10, 11, 12 and 13, the circumference of the take-up pinch roller 7 and the guide roller 14 to the take-up reel 4. The ink ribbon 3 is exposed at the print head receiving part 6. The ink ribbon 3 unwound from the feed reel 5 travels along the passage and is taken up on the take-up reel 4.
The respective circumferences of the guide rollers 11 and 12 disposed at the opposite ends of the print head receiving part 6 lie on the outer side of a plane including the front side of the housing 1. The ink ribbon 3 is extended between the guide rollers 11 and 12 with its ink-coated surface in contact with the circumferences of the guide rollers 11 and 12 as indicated by broken line when the ink ribbon 3 is of a hot-peeling type. The ink ribbon 3 is extended between the guide rollers 11 and 12 with its back surface opposite its ink-coated surface in contact with the circumferences of the guide rollers 11 and 12 as indicated by solid line when the ink ribbon 3 is of a cold-peeling type.
The ink ribbon 3 enclosed in the housing 1 may be a hot-melt color ink ribbon having a layer of a hot-melt color ink, such as hot-melt yellow, cyan, magenta or black inks, a metallic ink ribbon having a layer of an ink having a metallic luster, a volatile color ink ribbon having a layer of a volatile color ink, an ink ribbon having a layer of a transparent hot-melt ink for undercoat printing or overcoat printing or a lustrous ink ribbon for printing a lustrous image by printing an image and heating the surface of the printed image for smoothing.
The type of the ink ribbon 3 is identified by detecting a type mark, not shown, formed on the housing 1 by an ink ribbon identifier mounted on the carriage of the thermal transfer printer.
Since the tensile load is applied to the ink ribbon 3 by the agency of the tension shaft of the thermal transfer printer, the tensile load is kept constant for all types of ink ribbons regardless of type. Therefore, all types of ink ribbons are not necessarily able to move steadily. Different types of ink ribbons behave differently when used for printing on the thermal transfer printer. Therefore, printing conditions, such as pressure for pressing the thermal print head against the platen, mode of driving the heating elements of the thermal print head and printing speed, are controlled to print images properly. However, it is impossible to stabilize the movement of all types of ink ribbons only through the control of the printing conditions for the thermal transfer printer and, consequently, images are printed in a poor print quality.